Fluid control valves are used in a wide variety of applications for causing and controlling motion of various components. Hydraulic fluid control valves and systems are used in such applications when relatively large forces are to be transmitted and controlled through such hydraulic components.
One type of hydraulic fluid control valve is a sectional valve. A sectional valve may typically include a plurality of separate cast and machined metal working valve sections. Each working valve section may include internal fluid passages, external ports, and valve bores with valve members slidably disposed within each valve bore. The valve bores may include a main control valve spool bore in which a main directional control valve spool is slidably disposed, and a pressure compensator valve spool bore in which a pressure compensator valve spool is slidably disposed. In a pressure compensated working valve section the pressure compensator valve spool is arranged to maintain a predetermined pressure drop across a variable orifice of the main control valve spool under normal operating flow conditions independently of the inlet or outlet pressure. By maintaining a substantially constant pressure drop across the orifice, a substantially constant and repeatable flow rate through the orifice may be achieved. Commonly, the pressure drop may be controlled in part by the pressure compensator spool and by the force of a biasing spring acting directly or indirectly against the pressure compensator spool.
Pressure compensated working sections may also commonly include load sense passages. The load sense passages may be operably connected to provide (i.e., transmit) a pressure feedback signal from an outlet passage or work port, which indicates the fluid pressure required by a fluid operated device, such as an actuator, which receives flow from the sectional valve. The load sense passage may be operably connected to a variable displacement hydraulic pump or other source of pressure and flow to provide a feedback signal that communicates with the pressure compensator valve to control pressure and regulate fluid flow from the source.
During operational conditions, deadheading may occur in which a working section is provided with fluid pressure from the pressure source, but substantially no flow through the main flow control valve variable orifice occurs. Deadheading may occur, for example, when flow is directed toward an associated fluid receiving actuator and movement of the actuator in response to the flow is somehow restricted or stopped, for example, at the end of a cylinder's physical stroke, or by a load that is sufficient to resist further movement of the actuator. As the flow directed from the outlet passage or work port of the working section to the deadheaded actuator decreases substantially to about zero, the pressure in the working valve section may increase. As such, the working valve section may limit the fluid pressure at the work port by providing an associated pilot-operated pressure limiter valve in the flow path of the sectional valve.
A common pilot-operated pressure limiter valve maintains a spring biased check valve element against a valve seat in the flow path that is in fluid pressure communication with the work port. When the work port pressure becomes greater than a predetermined pressure that the spring holding the check valve element closed can support, the pilot-operated pressure limiter valve is activated to open a flow path enabling venting of fluid to a reservoir or tank whereupon the pressure compensator spool shifts towards the closed position, closing off flow to the main spool and creating just enough leakage past the compensator spool to maintain a pressure at which the pilot-operated pressure limiter valve was set. Therefore, the compensator spool becomes a pilot-operated pressure reducer to maintain working pressure at a desired level.
However, while such typical sectional control valves may accommodate the deadhead operating condition by providing such a pilot-operated pressure limiter valve, such systems may not be capable of accommodating for further increases in pressure in the valve section above the predetermined pressure limitation level set by the pilot-operated limiter valve. For example, when the fluid operated device is obstructed to the point where the device is deadheaded and then is driven in the reverse direction, causing fluid to flow back through the work port, the fluid pressure in the valve section may increase beyond the predetermined level set by the pressure limiter valve, which may cause damage or catastrophic failure of the sectional valve and associated components.